Speed Up Your Web Site with Varnish

System Load and %iowait

System load is a measure of how much load is being placed on your CPU(s). As a
general rule, you want the number to stay below 1.0 per CPU or core on your
system. That means if you have a four-core system as in the machine I'm
benchmarking here, you want your system's load to stay below 4.0.

%iowait is a measure of the percentage of CPU time spent waiting on input/output. A high %iowait indicates your system is disk-bound, performing many disk i/o
operations causing the system to slow down. For example, if your server had to
retrieve 100 files or more for each request, it likely would cause the
%iowait time to go up very high indicating that the disk is a bottleneck.

The goal is to not only improve response times, but also to do so with
as little impact on system resources as possible. Let's compare how a
prolonged traffic surge affects system resources. Two good measures
of system performance are the load average and the %iowait. The load
average can be seen with the top utility, and the %iowait can be seen
with the iostat command. You're going to want to keep an eye on both top
and iostat during the prolonged load test to see how the numbers change.
Let's fire up top and iostat, each on separate terminals.

Starting iostat with a two-second update interval:

iostat -c 2

Starting top:

/usr/bin/top

Now you're ready to run the benchmark. You want ab to run long enough
to see the impact on system performance. This typically means anywhere
from one minute to ten minutes. Let's re-run ab with a lot more total
requests and a higher concurrency.

Load testing Apache with ab:

ab -c 50 -n 100000 http://localhost/cgi-bin/test

Figure 6. System Load Impact of Traffic Surge on Apache

Load testing Varnish with ab:

ab -c 50 -n 1000000 http://localhost:6081/cgi-bin/test

Figure 7. System Load Impact of Traffic Surge on Varnish

First let's compare response times. Although you can't see it in the
screenshots, which were taken just before ab finished, Apache came in at
23ms per request (2097 rps), and Varnish clocked in at 4ms per request
(12099 rps). The most drastic difference can be seen in the load
averages in top. While Apache brought the system load all the way up
to 12, Varnish kept the system load near 0 at 0.4. I did have to wait
several minutes for the machine's load averages to go back down after
the Apache load test before load testing Varnish. It's also best to run
these tests on a non-production system that is mostly idle.

Although everyone's servers and Web sites have different requirements and
configurations, Varnish may be able to improve your site's
performance drastically while simultaneously reducing the load on the server.

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> It will consider only caching GET and HEAD requests. It won't cache a request with either a Cookie or Authorization header. It won't cache a response with either a Set-Cookie or Vary header

... which means that tools like Varnish are only useful to speed up sites that don't customize contents on a user-basis. I wonder how those other sites eg. Facebook handle the issue, since each page is different for each user.

This book is the material we use for our Varnish training classes and it gives a good introduction on the project history, what it is, how VCL works and how Varnish it is related to HTTP and Cache Invalidation.

This book is the material we use for our Varnish training classes and it gives a good introduction on the project history, what it is, how VCL works and how Varnish it is related to HTTP and Cache Invalidation.